The present invention relates to a method of authentication for Media Gateway, comprising: setting up an initial key for validating initial digital signatures between a Media Gateway and a Media Gateway Controller; generating a new shared key having a specific lifetime by performing signaling communication between said Media Gateway and said Media Gateway Controller with said initial key; authenticating calls and responses between said Media Gateway and said Media Gateway Controller with said new shared key; and updating said shared key between said Media Gateway and said Media Gateway Controller if the lifetime of said shared key is expired. The invention can authenticate each call, update the shared key periodically, and prevent calling invalidly effectively.